This invention relates generally to systems used to steer subsections of articulated vehicles and trailers and specifically to steering systems controlled by a microcomputer or a programmable controller to follow a lead subsection. The invention generally relates to articulated vehicles and trailers with three or more subsections. The invention also includes provisions for a plurality of operator selectable steering modes as well as a plurality of automatic steering modes.
The preferred embodiment of the invention demonstrates a way of applying the principles of the invention to dollies used in over-the-road tractor-trailer combinations. The desired steering axle angle is primarily a function of the angle between the steered section (the dolly) and the section in front of the steered section. Additional data and additional algorithms can be used by the software of the programmable controller to provide a refinement of the steering response.
Over-the-road transport companies find it difficult at times to compete with other freight haulers because of weight limits on the roads and bridges. Multi-trailer arrangements are a possible solution to some of these problems because they spread the load over a longer stretch of pavement and reduce the columnar loading on bridges. These arrangements generally involve a semi-trailer carried by the tractor with one or more full trailers composed of semi-trailers carried by xe2x80x9cdolliesxe2x80x9d.
The most common and widely used dolly is the standard Type A dolly, which hitches to the towing vehicle or first trailer using a single point hitch. The standard Type A dolly provides xe2x80x9cwagon tonguexe2x80x9d steering for the trailer it is carrying by allowing the entire dolly to steer relative to its semi-trailer about the fifth wheel vertical axis on the dolly as well as relative to the towing trailer about the single point hitch vertical axis. The dolly tires do not, however, steer relative to the dolly frame.
Commercial vehicles of either truck and full trailer or multi-trailer configurations which employ the standard Type A dollies generally possess undesirable characteristics such as limited maneuverability and instabilities caused by rearward amplification. Rearward amplification, sometimes described as a xe2x80x9ccrack-the-whipxe2x80x9d phenomenon, implies that in rapid evasive maneuvers such as emergency lane changes, the rearward elements of the vehicle train such as the dolly and the trailer carried by the dolly experience motions which are substantially amplified compared to the motions of the towing tractor and first trailer. Rearward amplification is known to be the basic cause of many accidents in which roll over of the last trailer or second trailer occurs while the remaining elements of the vehicle remain unscathed.
A second general class of dollies known as Type B dollies represent an improvement over standard Type A dollies. Type B dollies are generally characterized by a double tow bar arrangement which eliminates steering of the dolly with respect to the towing vehicle, most commonly the first trailer. The Type B dollies have been effective to a degree against some of the instability problems and are slightly more maneuverable than the standard Type A dollies. However, they cause other problems such as introducing other types of instabilities, causing stresses on the rear of the forward trailer, and increasing unloading delays due to difficulty in accessing the back of the forward trailer for some configurations.
Lengthy trailer combinations have always been plagued by these same two problems of instability and lack of maneuverability. Many transport companies have looked toward steerable dollies as a possible solution to these problems. Steering systems for dollies generally make use of Ackerman geometry and generally derive from modifications to the standard Type A dolly. With Ackerman geometry all tires of the dolly and the tires on the rear axles of both the first and any subsequent trailers are aligned tangent to circular paths which all have the same turn center. Ackerman geometry is the most desirable steering configuration for low speed maneuvering since it minimizes tire scuffing, wear and structural stress. With some modifications, Ackerman geometry can be adapted to provide stability when traveling at higher speeds, but this stability comes at the expense of maneuverability. An example of Ackerman geometry as applied to the steering of the rear tandem wheels of a semi-trailer is disclosed in U.S. Pat. No. 2,342,697.
The prior art teaches ratio steering systems for dollies with two principal steering modes. When the steering axle of the dolly is steered in a direction to make it initially more nearly parallel to the back of the trailer it is following, the behavior of the dolly and of the system as a whole tends to greater stability. This mode of steering could be called the stability type of ratio steering. This type of steering behavior is particularly desirable at higher speeds on the open road. When the steering axle of the dolly is steered in the opposite direction to the above, the dolly and the trailer that it carries are caused to swing wider around the corner, producing better maneuverability characteristics for the system as a whole. This mode of steering could be called the cornering type of ratio steering. This type of steering is particularly useful when maneuvering among obstacles or along curved streets at lower speeds.
These methods for automatically steering rear sections of articulated vehicles have all been based on a single basic algorithm: a certain angle change of the section in front of the steered section results in a certain change in the angle of the steering axle. This type of steering can be called xe2x80x9cratio steeringxe2x80x9d. In the mode in which the wheels are steered in the opposite direction as the section in front, a certain change in angle results in a change in the opposite direction, roughly corresponding to a ratio of 1:xe2x88x921 (one-to-minus-one). In general, as the ratio proceeds in the negative direction, i.e. 1:xe2x88x925, 1:xe2x88x9210, 1:xe2x88x9250, the turning radius is decreased. A type B dolly has a ratio corresponding to 1:negative infinity, causing an immediate extreme correction in the direction of rotation. As the ratio becomes more positive, the system becomes generally less maneuverable, with an increasing turning radius. A ratio of 1:0 corresponds to a standard a type A dolly. As the ratio increases toward 1:1, the system emulates a trailer with an increasingly long wheelbase. A mode in which the wheels turn in the same direction as the front section and have a ratio of 1:1 induces a crab type motion in which the entire system translates horizontally, but is incapable of changing direction. As the ration becomes greater, the trailer becomes unusable, with the rear trailer turning more than the front. Several patents have explored some of the possibilities for this type of ratio steering.
The present invention makes several contributions and extensions to the art of ratio steering for articulated vehicles, and particularly for dollies used in over-the-road freight transport. First, the present invention provides a method whereby the ratio to be used can be selected automatically by the software in the controlling microprocessor based on parameters such as vehicle turning angle or vehicle speed. At high speeds on relatively straight roads, the dolly operates in modes with positive steering ratios that are characterized by stability with rearward amplification substantially eliminated. When negotiating sharper turns at lower speeds, the dolly automatically switches to the cornering modes (negative steering ratios) that cause the dolly to swing wider around corners. These modes allow the dolly to maximize its maneuverability at low speeds. At intermediate speeds, a steering ratio will be chosen that is the best compromise of stability and maneuverability at that speed.
Second, the present invention provides for one or more additional corrections to the steering at a point later in the turn when the cornering type of ratio steering is ineffective. The overarching purpose here is to xe2x80x9ccorrectxe2x80x9d the path of the steered dolly so that it follows as close as possible the path taken by the tractor as it turned the corner. If the tongue of the dolly is long enough, these xe2x80x9coversteer correctionsxe2x80x9d will indeed allow the steering axle of the dolly to approximate a path that a tractor driver might take as he oversteers around the corner. In this case, then, the trailer carried by the dolly rounds the corner on a path that is reasonably close to the path taken by the first trailer. With this correction, the dolly that is an embodiment of the present invention substantially exceeds the cornering capabilities of the Type B dollies while maintaining superior stability when operating at higher speed on the open road.
All steering modes used by the present invention conform to a type of modified Ackerman geometry.